Json output

[aslakhellesoy]

This adds a --json option to the show and fmt commands. This opens up for advanced querying by piping to jq.

Examples:

# Get the latest released version
./changelog fmt --json | jq ".versions[1].name" --raw-output
0.7.0

# Get the date of a release
./changelog show 0.6.0 --json | jq ".date" --raw-output
2020-05-28

/cc @mattwynne

P.S. The build is failing, but that seems unrelated to this change.